Which projection best demonstrates the sacroiliac joints when the pelvis is obliqued by approximately 25-30 degrees?

• A. Lateral projection
• B. AP oblique projection in RPO
• C. AP axial projection
• D. AP projection with no obliquity

Answer: (B)

The key idea is that the sacroiliac joints are best seen when the pelvis is rotated about 25–30 degrees to open the joint spaces and reduce superimposition of the surrounding bones. An AP oblique view done with a 25–30 degree obliquity places the sacroiliac joint of interest in profile and closest to the image receptor, which provides the clearest delineation of the joint margins.

This is why the AP oblique projection in RPO is the best choice: the obliquity opens the SI joints and positions the joint of interest toward the IR for optimal visualization. Other projections either fail to achieve this obliquity or keep the bones too superimposed to clearly view the joint space—lateral views, AP axial views, or AP views with no obliquity do not provide the same unobstructed, well-profile view of the sacroiliac joints.